Vector control of the Aedes mosquito is challenging and dengue is seen as an emerging global threat.
Dengue largely affects the paediatric population but can present in adults. There is no human to human transmission.
- Incidence: 0.60 cases per 100,000 person-years
- Peak incidence: 20-30 years
- Sex ratio: 1:1
Aedes mosquitos are found throughout the tropics, which means dengue is endemic in many countries including:
- The Americas
- Western Pacific
- South-East Asia
The main vector for dengue is the Aedes aegypti mosquito. There are also some other Aedes species including Aedes albopictus which can transmit dengue.
The Aedes mosquito is typically found in urban settings, increasing human exposure and risk of contracting dengue. The mosquitos:
- Rest and breed indoors
- Lay eggs in containers e.g. tyres
- Very drought-resistant eggs
- Limited flight range (<100m)
- Prefer to bite humans
- Diurnally active
- Bed nets less effective
The major risk factors for developing dengue are:
- Visiting a dengue-endemic region
- High population density
- Poor standards of hygiene
Severe dengue is more likely to develop in:
- Children under 15 years old
- Repeated dengue infections
- Specific viral genotypes
- Malnourished children.
- Single-stranded RNA virus
- Within Flavivirus genus
- Has 4 different but closely related serotypes (DEN-1, DEN-2, DEN-3, DEN-4)
- May also be 5th genotype
- DEN-2 and DEN-3 (Asian serotypes) often associated with severe disease
- Especially if the dengue infection is a secondary infection.
Dengue has a 4-10 day incubation period after the bite from an infected mosquito. Viral multiplication occurs in macrophages, monocytes, and B cells. Can also happen in mast cells, dendritic cells, and endothelial cells. Patients can only transmit the infection via the Aedes for 4-5 days (maximum 12 days).
- No direct human to human transmission
Lifelong immunity to 1 specific viral serotype develops after infection. However, there is an increased risk of severe dengue in subsequent infections with different serotypes. This is called antibody-dependent enhancement.
- Unclear mechanism
- Only in 2-4% of patients
Initial presentation of dengue is with flu-like illness
- Fever typically starts on day 3
- Lasts for 5-6 days (viraemic phase)
- Can then recover or progress to severe dengue
- Mild haemorrhagic symptoms
- Dengue fever is rarely fatal
The major pathophysiological process in severe dengue is increased vascular permeability due to capillary leakage causing plasma leakage into tissues. This leads to:
- Cytokine response
- Suppression of T- cell response
The current classification system is the 2009 World Health Organisation scheme described below. This has superseded the previous classifications of dengue fever, dengue haemorrhagic fever and dengue shock syndrome.
1. Non-severe dengue: fever followed by recovery
Without warning signs
- Fever with two of the following:
With warning signs
- Abdominal pain or tenderness, persistent vomiting, clinical fluid accumulation, mucosal bleed, lethargy, restlessness, liver enlargement >2cm, increasing haematocrit with reducing platelets
The initial presentation of dengue is:
- Intermittent high pyrexias ‘break-bone fever’ lasting 3-7 days
- Typically blanching maculopapular erythematous rash similar to measles or scarlet fever
- May develop into petechiae
- Other bleeding manifestations including
- Bleeding gums
- GI bleeds
Other features may be quite non-specific and can include:
- Nausea & vomiting
- Generalised myalgia
- Ocular manifestations:
- Retro-orbital pain
- Conjunctival injection
2. Severe dengue (5% of patients) dengue with severe plasma leakage, severe haemorrhage and severe organ impairment.
Severe symptoms may include:
- Pulmonary and facial oedema
- Pleural effusions
- Meningism including photophobia
- Worsening or more profuse haemorrhage
Blood tests tend to show thrombocytopenia (up to 80%, although may be mild) and leucopenia (also up to 80%, tends to improve after day 5) for the majority of all patients (severe and non-severe). Haematocrit can be a useful monitoring tool, with increasing haematocrit often reflective of clinical deterioration.
Other bloods may show:
- Prolongation of APTT and PT
- Deranged U&E's
- Elevated LFTs especially AST
Diagnosis can be confirmed by:
- Viral isolation from serum
- Sample needs to be collected early during the viraemic period (before day 5)
- PCR (where available)
- Antibody detection using ELISA: IgM and IgG
- Can be used after day 5
- Form used in rapid testing kits
- Allow distinguishing of primary (first flavivirus exposure) from secondary (previously exposed to different flavivirus)
The tourniquet test can also be used but is only positive about 1/3 of patients, with a specificity of 71%. It can be used in low resource settings with limited access to serological testing.
- Inflate a BP cuff to halfway between systolic and diastolic pressure for 5 mins
- A positive test shows 20+ petechiae in a 2.5cm square on the forearm
Dengue often presents with a mild flu-like illness which may be difficult to separate from many other common viral illnesses in UK practice.
- Consider travel history and vaccination status
Common UK viral illnesses may include:
- Infectious mononucleosis
Common and important tropical infections to consider include:
- Joint swelling more prominent feature
- Often mild or no fever
- Purulent conjunctivitis
- Other viral haemorrhagic fevers e.g. Lassa fever, Ebola
- Often geographically localised, especially in West Africa
- Acute HIV seroconversion
- Consider risk factors for HIV e.g. high risk sexual activities
- Often presents with GI symptoms
- Rickettsia infections (typhus, scrub typhus)
- Consider water exposure
- Meningococcal septicaemia
- Consider immunisation history and contact exposure
- Autoimmune diseases (e.g. SLE)
- Adverse drug reaction
There is no specific treatment for dengue. Antivirals and steroids have not been shown to improve outcomes.
There is no direct human to human transmission so no requirement for isolation.
- Conservative treatment with oral fluid and paracetamol
- Avoid aspirin
- Increased haemorrhage risk
- Should also be avoided in children due to risk of Reye's syndrome
Severe cases (uncommon in returned travellers):
- IV fluids
- Used to restore circulatory volume due to plasma leakage into third space
- Regular observation and monitoring of haematocrit, platelets and renal function
- May require High Dependency or Intensive Care
- Severe GI haemorrhage
- Will require blood transfusion +/- FFP
- Use of personal protection equipment
- Long-sleeved clothing
- Vaccine but it is not currently recommended for use
- Potential to precipitate more severe cases of dengue in people who have been immunised.
- 50% mortality rate if untreated.
- Reduced to under 5% with treatment.
The usual mortality rate of dengue is <1%.
Patients can also develop:
- Hepatic failure
- Disseminated intravascular coagulation